align extent-api settings

today i ran in a situation that extent image shows an error

```
no protocol: settings
```

the problem was caused by:
- region/getmap/background setting had value "oceans" (from previous install)
- [config-spring-geonetwork.xml](https://github.com/geonetwork/core-geonetwork/blob/aa37f95246d2abae70e801c01dc33224daebef14/web/src/main/webResources/WEB-INF/config-spring-geonetwork.xml#L164) did no longer contain a definition for "oceans"
- [background-map setting](https://github.com/geonetwork/core-geonetwork/blob/aa37f95246d2abae70e801c01dc33224daebef14/web/src/main/webResources/WEB-INF/config.properties#L48) on config properties had no effect


- "Workaround", make sure region/getmap/background contains "osm" (or add a configuration in config-spring-geonetwork.xml)
- Basic improvement is to print in image a message that "oceans" is not one of available map configurations.
- Alternative is to remove the setting region/getmap/background and depend only on config.properties configuration

Start with web/src/main/webResources/WEB-INF/config-spring-geonetwork.xml and config.properties, then trace how region/getmap/background is used when generating the extent image. Reproduce the error with the "oceans" value and determine whether the intended result is a clear invalid-configuration message or consistent configuration precedence; verify the chosen behavior with a regression test.

Written by the indexing model from the issue text.